Resilient lapping element especially adapted for use in textile printing



" Dec. 6, 1960 A. w. NICONCHUK arm. 2,963,393

RESILIENT LAPPING ELEMENT ESPECIALLY ADAPTED FOR USE IN TEXTILE PRINTING Filed June 25, 1956 2 Sheets-Sheet 1 l4 I3 l2 l4 l3 I2 FIGURE! Dec. 6, 1960 A. w. NiCONCHUK E'IAL 2,963,393

RESILIENT LAPPING ELEMENT ESPECIALLY ADAPTED FOR USE IN TEXTILE PRINTING 2 Sheets-Sheet 2 Filed June 25, 1956 FIGURE 3 United States Patent RESILIENT LAPPING ELEMENT ESPECIALLY ADAPTED FOR USE IN TEXTILE PRINTING Alec W. Niconchuk, Peabody, and William C. Ross, Winchester, Mass, assignors to W. R. Grace & Co., Cambridge, Mass., a corporation of Connecticut Filed June 25, 1956, Ser. No. 593,401

1 Claim. (Cl. 154-54.5)

This invention relates to a new form of lapping material specifically designed for use with cylinder printing machines such as those commonly used for printing textiles.

More specifically, this invention relates to a laminated lapping element comprising a plurality of plies of a resilient, woven textile fabric which plies are joined together by interply layersof a rubbery material. The basic elements of a cylindric printing machine consist of a large, smooth cylinder and one or more small rollers, each arranged on an axis parallel to that of the cylinder and about the periphery of the cylinder. The impression to be printed is carried on the surface of the rolls usually in the form of an engraving, and normally there is a separate print roll for each color to be printed. The cloth to which the printing impressfon is to be applied passes around the cylinder beneath the rollers. There is, in addition, certain auxiliary equipment such as the drive mechanism. color feed, and the like, which although essential for the operation of the printing range, do not enter into the present invention.

Printing on cloth presents certain problems because of the inherently open and flexible structure of the cloth. The intensity of the color deposited on the cloth depends to a large degree upon the quantity of color applied and the extent to which this color penetrates the cloth. With a given engraving, both the quantity of color and the depth to which the color penetrates are to a large degree a function of the pressure exerted upon the cloth by the print roller.

It has been customary to employ several layers of material between the cloth and the print cylinder. A second layer of cloth, known as the backgrey, is run through the printing machine around the cylinder directly behind the cloth which is to be printed. The function ofthe backgrey is to absorbany excess color which may'pass through the layer of cloth to which the color is applied, thereby preventing ofiset.

A printing blanket is passed through the printing machine and around the cylinder directly beneath the backgrey. One function of the printing blanket is to provide a uniform support for the cloth while it is being printed. The backgrey and the printing blanket may be combined in a single structure known as a wash blanket, such as that disclosed in US. Patent 2,436,761 or in U.S. Patent 2,723,932.

Finally, in conventional practice, the surface of the cylinder is wrapped with several thicknesses of a special, highly resilient fabric known as lapping. The function of the lapping is to provide a resilient base for the printing.

' A proper degree of resiliency is an important factor to the successful operation of a textile printing machine. As mentioned above, the quality of the printed impression is dependent to a large degree upon the pressure with which the print roller presses on the cloth. Since, to make a satisfactory impression, the color must be driven into the cloth to some depth. textile printingat least on a cylinder machine-is always conducted with a considerable pressure exerted between the print rollers and ice the cloth. This pressure is achieved by forcing each print roller toward the cylinder by means of a jacking mechanism. In certain instances, this jacking mechanism may be hydraulic or may be spring-loaded, but most usually it is an ordinary screw jack arrangement.

Thus, when fitted with a screw jack, the printing machine relative to any single print roller consists of'two cylinders with parallel axes set apart at a fixed distance. In order to exert any pressure on the cloth, this distance, at'the point of closest approach of the two cylinders, must be less than the aggregate thickness of the layer of material passing between the two cylinders.

Under these conditions, the actual pressure exerted upon the cloth becomes an extremely critical function of the variation .in thickness of the material in the bite and'of the variation in radii of the two cylinders. Without the lapping, a decrease in the thickness of the bite dueto a slight degree of out-of-roundness of either cylinder, pro? vided the thickness of the material passing throughthe bite remains constant, could be relieved only by the deflection of one of the cylinders. The point pressure exerted upon the cloth under such circumstances would increase to that required to cause this deflection. The same result would be obtained if the thickness of the material passing through the bite between the two cylinders wereto increase. This could occur duetoaslub, for example, in the. cloth or the backgrey, to a variation in thickness or density of the blanket, or more drastically, to a hard scrimp or a foldover passing through the bite.

On the other hand, without lapping, a decrease in the thickness of the material passing through the bite or an increase in the distance between the adjacent surfaces of the two cylinders would result in a very drastically reduced degree of pressure on the cloth. Since the tonal values of the color and the quality of the printed impression are a function of the pressure at which the coloris applied, it is seen that the presence of lapping is important if uniform printing is to be achieved, since it is quite impossible to machine the cylinder or to manufacture the cloth or the backgrey or the blanket to absolute tolerances.

Lapping is equally necessary when a pressure-loaded (e.g., a hydraulic actuated or a spring-loaded) jacking mechanism is used. Most of the thickness variation in material passing through the bite between the print roller and the print cylinder occurs only over a minor portion of the full width of the cloth. Since the function of such a jacking arrangement is to relieve overpressure by permitting the print roller to move, a slub in the fabric would cause a light streak across a significant portion of the width of the fabric if lapping were not to be used.

As might be expected, the nature and the construction of desirable lapping fabrics have been well worked out over the years. -Most usually such a fabric is formed of heavy yarn in a twill or square construction. Since it is important that the fabric be dimensionally stable, at least in the longitudinal direction, a strong, long-staple fabric such as cotton or linen is normally employed for the warp yarns. The lapping cloth is woven to keep the warp yarns as straight as possible. Since the resilience in such an instance is provided essentially solely by the fill, the fill is formed from yarns of a kinky and resilient fiber such as wool. The fill yarns are formed in such a way as to preserve as much of the natural resilienceof the fiber as possible. In this way a very strong, yet highly compressible fabric is obtained. v

The proper use of lapping fabrics requires a high degree of skill. (See, e.g., The Principles and Practice of Textile Printing, Knecht & Fothergill, Fourth ed., emended by J. G. Hurst, Charles Griffin & Company, Ltd., 1952, pps. 62-63.) The overall thickness of the several layers of lapping fabric wrapped about the'cylinder must be kept constant, for the slightest crease or sign of an overlap may cause misprints on the cloth. The tension on the lapping fabric throughout the full length wrapped around the cylinder should be kept uniform.

Even in spite of the exercise of the highest degree of skill, the use of lapping has never been completely satisfactory. The resilience of a bed of lapping varies continuously throughout the entire life of the lapping. When the lapping is first applied, the bed of lapping is very soft and resilient. As it is used, the lapping bed becomes progressively harder and less resilient until finally it must be removed because it lacks sufficient resilience to perform properly. This hardening effect is accelerated drastically if the lapping bed becomes wet. This condition occurs quite frequently when the cylinder side of a wash blanket becomes wet at the edges due to improper or incomplete drying after washing. The moisture transferred to the lapping in this way may travel across a significant portion of the lapping bed due to a wicking action. In addition to the loss of resiliency, the lapping fabric, even in spite of the attempt to make it as dimensionally stable as possible in the warp direction, continually stretches during use. This may require that the lapping be rewound about the cylinder from time to time. Winding and rewinding is an expensive operation, since it is time consuming and it puts the printing machine out of operation for the time.

Skill is further required since, under normal practice, different types of impressions require different printing pressures and hence in most instances, for reasons which are explained below, require different degrees of resiliency in the lapping. For example, as a normal rule large heavy patterns with deep engravings require a low degree of pressure and a very soft and resilient lapping. On the other hand, light and delicate designs such as shirting, for example, require a high degree of pressure and a hard lapping. For this reason, the type of printing that can be performed on any single machine depends to a large degree upon the condition of the lapping. The textile printer must pick his patterns for each machine. Indeed even on moderately long runs of a single pattern, the printing quality may deteriorate as the lapping hardens, requiring either the renewal of the lapping or a change of printing machines.

On the other hand, it has been noted that during a short portion of the life span of many lappings, it is possible to print almost any pattern using the appropriate pressure for each pattern without difliculty. As far as we can determine, the limited usefulness of most lapping is due to a combination of both the resiliency and the stretchability of the lapping fabric. Of the two, the stretchability is the more important factor in limiting the usefulness of the lapping in the early stages, and the loss of resiliency is the more important factor in limiting the usefulness of the lapping in the later stages.

Once the lapping has been run in enough to lose its extreme initial softness, the resilience seems to be about right for practically all printing. It further appears that the difiiculty in printing patterns which require a high printing pressure is due to the fact that the lapping fabric still retains the ability to stretch in the warp direction. This is explained by the fact that if any ability to stretch under the pressure employed remains in the lapping fabric, the fabric will increase in length. An increase in length will result in a loss of tension in the lapping fabric about the cylinder. Loss in tension then permits what is tantamount to a standing wave of lapping fabric to form immediately before each printing roller as the printing cylinder revolves. We believe that it is this standing wave that renders it impossible to obtain good printing impressions at the pressure needed for the printing of fine delicate patterns, and not the resilience or softness of the lapping itself. It further appears that with time the several layers of lapping fabric rearrange their relative position in a direction towind up more tightly about the cylinder and thereby restore the necessary tension. Apparently the temporary stage, which sometimes occurs where a wide variety of patterns can be printed on a single lapping, occurs when the tension on the lapping is restored by this rearrangement before the lapping loses as appreciable amount of its resilience.

We have discovered a way to construct a lapping element consisting of a plurality of plies of a lapping fabric with interply layers of a rubbery material, which element has the necessary characteristics to make it possible to print every type of pattern throughout the normal range of printing pressures interchangeably. This new lapping element further retains these desirable characteristics substantially without change throughout its entire working life. This remains true even when the lapping-element is run in the presence of water. This working life is much longer than that normally expected for lapping material. in fact, it appears that the only limitation on the usable life of our new lapping element lies in the abrasion resistance of the surface plies of lapping fabric and that failure occurs only when the surface layers have physically worn away.

Our invention is best understood by reference to the drawing, wherein:

Figure l is a diagrammatic view of a printing machine showing particularly the relation of our lapping element to the conventional parts of such a machine,

Figure 2 is a diagrammatic cross section of our lapping element, and

Figure 3 is a chart showing the relation between compressibility and pressure of our lapping element and showing particularly the upper and lower limits of this relation.

In Figure l the printing cylinder is represented at 11 and the printing rollers are represented by the numbers 12. The ink supply and the ink pan for each printing roller 12 are represented by the numbers 13 and 14, respectively. The textile to be printed is represented by 1.5, the wash blanket by 16, and the lapping element by 17. The lapping element 17 passes around printing cylinder 11 and around idler roll 18. Idler roll 18 is adjustable by conventional means (not shown) relative to the axis of printing cylinder 11. Guide rollers for cloth 15, wash blanket 16, and lapping element 17 are indicated at 19, 20, and 21, respectively.

A cross section of a typical lapping element 17 according to this invention is shown in Figure 2. In the particular lapping element illustrated, there are six plies of lapping material 22 separated and bonded together by interply layers 23 of rubbery material.

The lapping element is manufactured in the following manner. First, the lapping material 22 is selected from conventional lapping materials on the basis that it exhibits the following properties:

(a) A coarse weave with a raised but unnapped surface, preferably but not necessarily a twill;

(b) A minimum stretch with a maximum tensile strength;

(c) A maximum resilience without sacrifice to other desirable properties.

A fabric of this general nature which we have been able to employ with great success is a plain twill lapping fabric supplied by E. H. Best & Co. This is a cotton and wool plain twill weave having cotton in the warp and wool in the fill. On a total weight basis it is 60% cotton and 40% wool. The thread count is 54 in the warp and 34 in the fill, using a 10/3 thread in the warp. The weight is 1.05 pounds per square yard with a thickness (overall) of 0.038 to 0.040 inch.

Second, for the interply layer of rubbery material, we prefer to employ a liquid rubber composition. The requirements of such a composition are:

(a)- That it have a sufiiciently high viscosity so that there is a minimum amount of penetration into the fabric;

(b) That the compound upon drying and curing will Parts by weight Neoprene latex (#571) 415 Wetting agent (Emulphor OH) 2.1 Sodium silicate 3.3 Ball mill batch 112 Methylcellulose 8.3 Anti-foam agent 0.1 Water 777 This composition is prepared in the normal manner in the liquid phase. The ball mill batch referred to is prepared by mixing the following ingredients in a ball mill according to conventional practice until a smooth dispersion results:

Parts by weight Zinc oxide 90 Antioxidant (Agerite powder) 36 Retarder (Altax) 18 Calcium carbonate 90 Iron oxide, pigment grade 4.5 Dispersing agent (Daxad 11) 4.5 Bentonite clay 2.5 Water- 195.5

A thin coating of this composition is spread coated on each side of each ply of lapping fabric with the exception of the two outside plies. We prefer to coat the latter two plies with the composition on the inner side, so that when the lapping element is assembled, the outer surfaces will i be free from rubber composition. Using this particular interply composition, we prefer to coat each side of'each ply with about 30 to about 90 pounds (dry solid content) of composition per hundred square yards of lapping fabric and preferably with between 50 and 60 pounds (dry solid content) of composition per hundred square yards of lapping fabric. After coating, the composition deposited on the surface of the lapping fabric is thoroughly dried to remove the liquid content of the composition, which of course in this case is water.

Since the dried film of this particular composition has a density of approximately 86 pounds per cubic foot, this quantity of compound results in a film, assuming the composition to be spread on a plane surface of between 0.005" and 0.015" and preferably between about 0.007" and 0.010" thick. For comparison, 50 pounds per 100 square yards corresponds to a thickness of 0.0077" and 60 pounds to 0.0094". Should a different composition be used, the quantity applied should be adjusted to obtain a film of corresponding thickness. Of course, the film of rubbery interply material does not lie in a plane surface but rather follows generally the irregular surface configuration of the coarse lapping fabric, providing, however, a substantially continuous coating over the entire surface of the lapping fabric.

Due to the viscosity or, perhaps more properly, the plasticity of the coating composition, there is very little penetration of the composition into the lapping fabric itself. In fact, when using the preferred range of 50 to 60 pounds (dry solid content) of composition per 100 square yards and using the preferred pressure during lamination (described in detail below), it has been noted that even a slight nap on the fabric will destroy the bond, since the interply laminating composition adheres to the nap and not to the fabric itself. For this reason, we prefer to use an unnapped fabric.

After the interply coating composition is thoroughly dry, the several plies are assembled. We prefer to form each ply from a separate length of lapping fabric and to form an endless lapping element by splicing the several plies according to the method disclosed in Merrill, US. Patent 2,547,220, dated April 3, 1951.

Finally, in the presence of a sufficient degree of heat for a sufiicient time to cure the rubber, the assembled plies are then subjected to a limited degree of pressure in a conventional press to form a consolidated laminate. Typical conditions for this press curing step include a platen temperature of 320 F a pressure of 20-25 pounds per square inch gauge, and a time of 20 minutes.

The laminated lapping element is then ready for use. As disclosed above, we prefer to have this lapping element pass around print cylinder 11 and around idler roll 18. Idler roll 18 is adjustable relative to print cylinder 11 in order that the lapping may be retained under ten sion at all times. While it is preferable to place the laminated lapping element directly in contact with the surface of print cylinder 11, such elements have lbeen'installed where the surface of print cylinder 11 retains a plurality of windings of used and hardened lapping fabrics with satisfactory results.

Three factors enter into the construction of a suitable lapping element under the conditions outlined above. They are: (a) the number of plies, (b) the thickness of the film of interply coating composition deposited upon each ply, and (c) the pressure under which the lamina tion is carried out. The other conditions being equal, the overall resilience of the lapping element is directly proportional to the number of plies in the element. We have found that a laminated lapping element having six plies of the lapping fabric described above is suitable for general printing conditions. This being so, the following discussion is all relative to such a structure. It should be understood, however, that the results achieved in any particular case can be varied by varying the number of plies of lapping fabric and that if the number of plies is varied, the actual results obtained can be expected to be in direct proportion to the number of plies actually'employed as compared to six.

The second variable, the thickness of the film of the interply laminating composition, has been explained in some detail above. A film having a thickness less than that of the preferred range of 7-10 mils generally results in the loss of interply adhesion until at below about 5 mils this adhesion is reduced to a point wherein a lapping 1 element results which is liable to delaminate under printing pressures. If a film thicker than about 10 mils is employed, there is no increase in interply adhesion. However, there may be a decrease in resilience unless the pressure under which the laminate if formed is reduced. Above about 15 mils thickness, it is difficult to obtain a lapping element of sufiicient resilience even at the lower pressures.

The third factor, the laminating pressure, is perhaps the most important. It is the pressure that is the prime factor within the limits outlined above in establishing the compactness or density of the laminates and hence the resilience or compressibility of the laminates under varying degrees of pressure. a

Before we describe this factor in detail, it should be understood that under the pressures employed in forming the laminate, the several plies of lapping fabric remain resilient. Therefore, the reduction in the thickness'of the unconsolidated laminate during consolidation is a direct function of the laminating pressure. Thickness is easier to control than pressure at the low pressure necessary to construct a suitable laminate. Therefore, we prefer in commercial practice to control the thickness by using incompressible side packs, or other equivalent means to limit the maximum approach of the platens of the press. When side packs are used, the pressure becomes unimportant, of course, except that the pressure must be at least equal to that which would result in the desired thickness were the side packs not used.

To determine the effect of pressure, we consolidated eight identical unconsolidated 6-ply laminates made according to the procedure outlined above, using 50 pounds (dry weight basis) per hundred square yards of interply composition at a temperature of 320 F. for 20 minutes and at various pressures. The initial thickness of the unconsolidated laminate was 0.320". The results were The compressibility was measured as a loss in thickness of the finished sample when subjected to a pressure of 500 pounds per square inch gauge.

To determine the suitability of these various samples for use as lapping elements, the following simulated printing test, which shows excellent correlation with actual printing conditions, was conducted. The sample of lapping element was placed upon the bottom platen of a normal two-platen flat bed press. A section of a conventional rubber-faced textile wash blanket was placed above the lapping element sample. A piece of carbon paper with its carbon surface facing upward was placed on top of the wash blanket and a piece of white paper was placed on top of the carbon paper. A small piece of masking tape in either single or double thickness was placed on the under side of the wash blanket adjacent to the sample of lapping element. The masking tape had a thickness of 0.007, and therefore, the double thickness of masking tape had a thickness of 0.014". An impression was obtained for each sample with a single and, in most cases, with a double thickness of masking tape at a pressure of both 300 and 500 pounds per square inch gauge on the assembly.

In each case it was noted whether the piece of white paper showed any sign of markotf caused by the pressure of the small piece of masking tape. This could be noted by the difference, if any, of the density of the impression made by the carbon paper on the white paper between the area in which the masking tape was located and the remainder of the sheet.

The significance of the pressures chosen in this test was established by inquiries at various textile printing establishments. Under current practice, using normal lapping, it was determined that the lightest set or printing pressure used is rarely less than about 150 pounds per square inch, and the heaviest set or printing pressure rarely exceeds about 500 pounds per square inch. Most normal textile printing is conducted at about a 300-pound-persquare-inch pressure on the printing roll. Thus the pressures selected above represent normal conditions and extreme conditions of pressure.

Sample 4, which was consolidated at a pressure of 89 pounds per square inch, represents about the least amount of compressibility that would be expected to be satisfactory in an actual printing operation. A thickness variation at the nip of 0.014 is more severe than would normally be expected in an actual printing operation, whereas a thickness variation up to about 0.007 inch is difiicult to eliminate. On the other hand, the range of compressibility represented by Sample 1, which was cured at 22 pounds per square inch, would appear to be about the maximum compressibility that would have any general utility, although lapping elements having a greater degree of softness might be useful for the lighter printing pressure conditions.

A comparison of the compression characteristics at various pressures of the two extremes is shown in Figure 3. The compression characteristics of Sample 1 are shown in curve A, and the compression characteristics of Sample 4 are shown in curve B. Intermediate laminating pressures between those of Sample 1 and Sample 4 result in compression characteristics between curve A and curve B on Figure 3, as might be expected. The compressibility curves at various pressures of lapping elements laminated at intermediate pressures all have similar characteristics to curves A and B. We prefer to construct our lapping element, if it is intended to be used over a wide range of printing conditions, under conditions which result in the compression characteristics more like those of curve A than curve B.

In the range of laminating pressures between and including the pressures used to laminate Sample 1 (curve A) and Sample 4 (curve B), the lapping element has a very interesting structure. As explained above, lapping fabric preferably is woven so that the warp yarns are essentially straight and so that the thickness of the fabric is achieved by the resulting distortion of the fill yarns. Examination of lapping elements Within the preferred range reveals that substantially all of the bond between plies occurs between adjoining fill yarns and that substantially none of the bond between plies occurs between warp yarns.

The bond appears as columns of the rubbery interply layer surrounded by air spaces or unbonded areas if the plies are forcibly separated. In other words, the interply layer does not appear to be continuous but rather is discontinuous with areas of bond surrounded by areas in which there is no bond and probably not even any contact between adjoining portions of the interply layer deposited upon each ply.

An increase in laminating pressure results in an increase of the area of bond and a proportionate decrease in the area of no bond between each ply until above about the laminating pressure represented by curve B (e.g., 89 pounds per square inch) the areas without bond become substantially less than the areas of bond and a substantial proportion of direct bonding between the warp yarns of adjoining plies occurs. Bonding between the warp yarns of adjoining plies apparently limits the degree of recovery of the laminate after lamination and hence limits the degree of compressibility remaining in the laminate.

On the other hand, at a given laminating pressure, a thicker layer of interply rubbery material on the surface of each adjoining ply increases the tendency to form bonds through the interply layer between adjoining warp yarns. This apparently explains why, as the thickness of the coating on each ply increases above the preferred range, the maximum pressure at which a suitable laminate can be obtained is reduced.

In summary, therefore, a comparison of visual observation with the results under printing conditions reveals that satisfactory results are obtained when the majority of bonding between the several plies of lapping fabric occurs between adjoining fill yarn and when very little, and preferably none at all, of the bonding between adjoining plies occurs between the warp yarns of the adjacent plies. As might be imagined since the fabric plies present an undulated appearance due to the above mentioned distortion of the fill yarns, practically all of the bonding occurs between adjoining high-points of such yarns on adjoining surfaces of adjoining plies.

Satisfactory results have been obtained when the unconsolidated laminate has been laminated at a pressure between about 20 pounds per square inch and about 90 pounds per square inch. Since as explained above, under these circumstances an increase in pressure corresponds almost directly to a decrease in final thickness of the consolidated laminate, this may also be expressed, as satisfactory results have been obtained when the thickness of the consolidated laminate is between about 15% and about 25% less than the thickness of the unconsolidated laminate.

Extensive tests of our new lapping element under actual printing conditions on commercial textile printing machines have provided interesting results. Some of these may be summarized as follows:

(a) Laminated lapping elements which have been in use for at least twice the normal expected life of normal lapping show no change in impression characteristics (i.e., resiliency) other than that which might be anticipated due to the loss in thickness caused solely by the wear of the surface plies.

(b) A single laminated lapping element can be employed satisfactorily to print under a wide range of varying printing conditions, including difierent patterns, different fabrics, different engravings, difierent printing pressures, and may be so employed without concern for sequence. For example, a pattern requiring a light set may follow a pattern requiring a heavy set on a given printing machine without any effect on the quality.

A single laminated lapping element can be used to print a succession of difierent widths of cloth using corre- 10 spondingly diiferent widths of print rollers in any succession without difliculty.

(d) The recovery rate of the laminated lapping ele ment is sufficiently rapid to permit the use of a light-set printing roller immediately following a heavy-set printing roller during a single printing operation with satisfactory results.

(e) The presence of moisture on the laminated lapping element even for extended periods of time does not afiect its resiliency.

We claim:

A laminated, resilient structure having a controlled degree of compressibility and suitable for use as a lapping element in textile printing, comprising a plurality of plies of a resilient, woven, unnapped cotton textile fabric having resilient wool filling yarns, and wherein the resilience of the structure is supplied essentially by the wool filling yarns, said filling yarns defining spaced apices over the surface of said plies, said plies being bonded together by an interply layer of a rubbery material between each ply, wherein the major portion of bonding secured by the interply layer between adjacent textile plies having alternate converging and diverging apices is between the aligned converging apices of the filling yarns of adjoining surfaces of the adjoining textile plies, and the minor portion of bonding is between the warp yarns of adjoining textile plies.

References Cited in the file of this patent UNITED STATES PATENTS 1,639,218 Ebersol Aug. 16, 1927 2,031,013 Standish Feb. 18, 1936 2,164,499 Coughlin July 4, 1939 2,547,220 Merrill Apr. 3, 1951 2,723,932 Ross et a1. Nov. 15, 1955 2,743,206 Verduin Apr. 24, 1956 

